Because of its low density, most of the gases have been stored at high pressure or at low temperature as liquid cryogenic. When the gas is stored as a cryogenic liquid and it is going to be used as a gas at environmental temperature, the refrigeration delivered is not recovered. For this reason, when the tank containing the liquid cryogenic is going to be refilled, it is necessary to produce more cryogenic liquid using refrigeration systems. Additionally, because the liquefaction plant is not always at the same location where the gas is going to be used, it is often necessary to transport the gas in special trucks and to store the gas in cryogenic tanks. For this reason the cost of storing and transporting gas can be very high and the applications of some gases such as Natural Gas and Hydrogen have been limited.
For example, because of its clean burning qualities and convenience, natural gas has become widely used in recent years. However, many of the sources of natural gas are located in remote areas, great distances from any commercial markets for the gas. Sometimes a pipeline is available for transporting produced natural gas to a commercial market. When pipeline transportation is not feasible, produced natural gas is often processed into liquefied natural gas (which is called “LNG”) for transport to market.
A commonly used technique for non-pipeline transport of gas involves liquefying the gas at or near the production site and then transporting the liquefied natural gas to market in specially-designed storage tanks aboard transport vessels. The natural gas is cooled and condensed to a liquid state to produce liquefied natural gas at substantially atmospheric pressure and at temperatures of about −162° C. (−260° F.) (“LNG”), thereby significantly increasing the amount of gas, which can be stored in a particular storage tank. Once an LNG transport vessel reaches its destination, the LNG is typically off-loaded into other storage tanks from which the LNG can then be vaporized as needed and transported as a gas to end users through pipelines or the like.
Typically, LNG refrigeration systems are quite expensive because so much refrigeration is needed to liquefy natural gas. Natural gas, which is predominantly methane, cannot be liquefied by simply increasing the pressure, as is the case with heavier hydrocarbons used for energy purposes. Although, methane can be liquefied at temperatures of about −162° C., methane gas can only be liquefied below its critical temperature regardless of the pressure applied. The critical temperature of methane is about −82.5° C. (−116.5° F.). In addition, since natural gas is a mixture of gases, it liquefies over a range of temperatures. For example the critical temperature of natural gas is between about −85° C. (−121° F.) and −62° C. (−80° F.). Typically, natural gas compositions at atmospheric pressure will liquefy in the temperature range between about −165° C. (−265° F.) and −155° C. (−247° F.). One of the significant costs involved in liquefying natural gases is the cost of the refrigeration equipment including the refrigeration material.
Although many refrigeration cycles have been used to liquefy natural gas, the three types most commonly used in LNG plants today are: (1) “expander cycle” which expands gas from a high pressure to a low pressure with a corresponding reduction in temperature, (2) “multi-component refrigeration cycle” which uses a multi-component refrigerant in specially designed exchangers, and (3) “cascade cycle” which uses multiple single component refrigerants in heat exchangers arranged progressively to reduce the temperature of the gas to a liquefaction temperature. Most natural gas liquefaction cycles use variations or combinations of these three basic types.
For example, the cascade system generally uses two or more refrigeration loops in which the expanded refrigerant from one stage is used to condense the compressed refrigerant in the next stage. Each successive stage uses a lighter, more volatile refrigerant which, when expanded, provides a lower level of refrigeration and is therefore able to cool to a lower temperature. To diminish the power required by the compressors, each refrigeration cycle is typically divided into several pressure stages (three or four stages is common). The pressure stages have the effect of dividing the work of refrigeration into several temperature steps. Propane, ethane, ethylene, and methane are commonly used refrigerants. Since propane can be condensed at a relatively low pressure by air coolers or water coolers, propane is normally the first-stage refrigerant. Ethane or ethylene can be used as the second-stage refrigerant. Condensing the ethane exiting the ethane compressor requires a low-temperature coolant. Propane provides this low-temperature coolant function. Similarly, if methane is used as a final-stage coolant, ethane is used to condense methane exiting the methane compressor. The propane refrigeration system is therefore used to cool the feed gas and to condense the ethane refrigerant and ethane is used to further cool the feed gas and to condense the methane refrigerant.
Accordingly, what is needed is a system and method of storing gases at low temperature using a cold recovery system where materials further known as cold recovery materials are subsequently cooled by expansion or vaporization of the previously stored liquefied gas or low temperature gas. In this way the cold recovery materials will have the required temperature to cool the gas when the tank is refilled.